The present invention broadly relates to a new and improved method of and apparatus for measuring rheological characteristics of substances in a rotary viscometer.
In its more specific aspects the present invention relates to a new and improved construction of a rotary viscometer in which a measuring or measurement element or probe immersed in a measurement or measuring vessel containing the substance to be investigated, and in which either the measurement vessel or measuring element is turned or angularly displaced about a predetermined vertical axis by means of an associated drive member. At least the measureing or measurement vessel and the measuring or measurement element conjointly constitute a measuring arrangement.
In other words, the viscometer of the present invention is of the type comprising a first drive member such as an electric motor, a measurement or measuring vessel driven by the first drive member, a measurement or measuring element or probe arranged in free suspension in the measurement vessel by means of a torsion wire and a shaft, a correspondingly associated measurement or measuring system and a second drive member associated with the shaft. The second drive member is operatively connected to a reference value transducer or signal transmitter via a signal conductor. A regulation connection connects the measuring system with the first drive member for selectively regulating the angular velocity or angular displacement of the measurement vessel.
Generally speaking, the method of the present invention is for measuring the rheological properties of a substance in a rotary viscometer and comprises the steps of: depositing the substance to be measured in a measurement vessel; immersing a measurement or measuring element or probe in the substance in the measurement vessel such that the measurement vessel and the measuring element or probe conjointly form a measurement arrangement; and rotating or turning a selected one of the measurement vessel and the measuring element or probe about a substantially vertical axis by means of a therewith associated drive member.
A viscometer of this general type is, for instance, described in the German Petty Pat. No. 7,637,450, wherein determination of the flow or viscosity curve is achieved by incrementally changing the rotational speed of a geared motor driving the measuring or measurement vessel and by determining the amount of torque exerted on the measuring body and its suspension by the substance to be measured. The incremental change of the rotational speed or the rotational deflection of the measuring vessel will lead to inaccurate or incomplete determination of the flow or viscosity curve for certain structurally viscous substances.
As a function of structurally viscous characteristics, particularly within low viscosity ranges, e.g. in rheological blood tests, the characteristics of certain substances suddenly and irreversibly change when exceeding a certain predetermined rotary speed or angular velocity (shear rate), such that a given measurement cannot be repeated on the same sample for determining the exact crossover value or flow limit of the substance. Furthermore, the incremental change of the rotational speed of the measurement or measuring vessel is relatively inaccurate with equipment generally available at reasonable costs, such that even for small rotational speed increments it is difficult to determine the exact speed corresponding to a given torque (shear stress) correlated to the specific properties of the substance to be measured.